The present invention relates to a refractory material and in particular refractory material suitable for investment casting as well as non-foundry applications.
Investment casting is commonly used to produce high-quality cast products. Generally, an investment casting of a part is made by coating a pattern with a material that hardens to create a unitary, thin-walled, heat-resistant shell. The internal pattern may be constructed of either a low melting point substrate or a combustible substrate such as, wax, polystyrene, plastic, synthetic rubber, or any other substrate as is known in the art. Multiple such patterns may be attached to a common sprue providing an inverted tree like structure of the same volatizable material having one pattern attached to the end of each branch.
After the outer shell has dried around the one or more patterns, the patterns and sprue may be removed by melting, burning or the like, permitting a pattern with undercut portions to be removed from a single piece shell without damage to the surrounding shell. In this pattern removal process, the shell is heated to at least the melting or burning point of the pattern substrate and the pattern substrate is melted or burned away leaving only the shell and any residual substrate. The shell is then heated to a temperature high enough to flash off the residual substrate that remains in the shell.
Once the pattern substrate has been removed from the shell, a hollow central channel with tubular branches leading to cavities corresponding to the one or more patterns will remain. A casting material, for example molten metal, may then be poured down the central channel to pass to the branches into each of the cavities. When a casting material has hardened, the shell material is removed and the cast parts separated from the sprue.
One process of forming the shell used in investment casting involves the repeated steps of dipping the pattern in a liquid slurry, coating the dipped part with refractory material such as stucco, and allowing the slurry and refractory material to harden between dips. Through this repeated process, shells are gradually built up to a thickness of approximately ⅛″ or more. It is not uncommon in the industry to use seven or more layers per shell.
Overall, this process of shell building is time consuming because each coat of slurry, each with a corresponding coat of refractory material, must be air-dried prior to the application of subsequent coats. In many cases, shells cannot be baked at elevated temperatures for extended periods of time without compromising the integrity of the cured shell. In these cases, each cycle of dipping and air-drying may require as little as 1 to 2 hours for some patterns or as long as twenty-four to forty-eight hours for other patterns, resulting in a total time to produce a 5-layer shell that may be as short as 5 hours but may typically take several days.
The shell material must be not only sufficiently strong to resist the pressure of the molten cast material but it must also resist deformation, cracking, and/or excessive outgassing when in contact with high-temperature molten casting materials. Desirably the chemicals within the shell should not be overly reactive with the cast material. For example they should not react with the molten metal to produce oxides or scaling that will need to be removed.
The shell's slurry and refractory materials typically forms various chemical bonds as each layer hardens between dips. The type of chemical bonds formed within the investment casting shell depends on the ingredients used to form that shell, and can greatly influence the performance of the shell during casting. Refractory materials such as calcium aluminate cement, colloidal alumina and colloidal silica form hydraulic bonds, as is typical with most known forms of cement, which harden with the addition of water. Other refractory materials, such as sodium silicate and potassium silicate may form silicate bonds. Additionally, additives such as phenolic resin may be added to provide resin bonding while polyvinyl alcohol or carboxymethyl cellulose may be used to provide adhesive bonding in the shell. Once these materials are hardened and the shell is then heated, the refractory materials may also form ceramic bonds through the process of sintering.
However, unlike these prior refractory materials that are used in investment casting, it is desirable to form a refractory material capable of forming a shell for use during investment casting that provides increased structural integrity during casting and improved frangibility during shell removal.